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doi:10.2204/iodp.proc.330.106.2012 SedimentologySediment recovered from Hole U1375A, which penetrated to only 10.11 mbsf, represents a younger pelagic cap and older sedimentary cover of Achernar Guyot. No significant amount of sediment was recovered from Hole U1375B, except for a very small amount of foraminiferal debris recovered from the core catcher of Core 330-U1375B-1R (see “Paleontology”). Two stratigraphic units were defined in Hole U1375A on the basis of the compositional and textural characteristics of the sediment at macroscopic and microscopic scales (Fig. F3):
Unit I
Sedimentary observation for the uppermost part of Achernar Guyot (8.50 mbsf and above) was limited to a few cuttings that accumulated in the core catcher during retrieval of Core 330-U1375A-1R. The composition and age of the cuttings (see “Paleontology”) suggest that the sediment is similar to that forming the young unconsolidated pelagic caps observed at Sites U1372 and U1374 on Canopus and Rigil Guyots, respectively. Therefore, stratigraphic Unit I at Site U1375 is taken to represent the youngest sedimentary cover or pelagic cap. Its lower boundary at ~8.50 mbsf is inferred to correspond to a 2 cm thick ferromanganese encrustation on top of underlying Unit II (see below), which we interpret to represent the first occurrence downhole of a consolidated deposit (Fig. F3). The sediment of Unit I is composed of pale yellow unconsolidated sandy foraminiferal ooze with glass fragments. Its age is constrained by foraminiferal and nannofossil assemblages from the late Miocene to Holocene (see “Paleontology”). Smear slide observations (Sample 330-U1375A-1R-CC-PAL-SED, 0–2 cm) showed that the sandy foraminiferal ooze includes some fragments of fresh volcanic glass and pyroxene. Unit II
Stratigraphic Unit II represents an older, 1.61 m thick sedimentary cover extending from 8.50 to 10.11 mbsf. Unit II reaches to the bottom of Hole U1375A, and its lower boundary could not be defined because drilling was abandoned early as a result of hole instability. Its upper boundary is defined by a 2 cm thick ferromanganese encrustation at 8.50 mbsf. Unit II is composed of lithified basalt conglomerate or breccia. Distinct sedimentary compositions, textures, and fossil contents allowed Unit II to be divided into two subunits (Fig. F3). The high induration and consistent sedimentary textures in Subunit IIA and the occurrence of stronger alteration and the softer character of sediment in Subunit IIB (see “Alteration petrology”) suggest that the poor recovery of Core 330-U1375A-2R (49%) is probably related to disaggregation during drilling of Subunit IIB. The basalt clast types of Unit II are provided in Table T2 and in U1375A.DOC in CHAR in SEDIMENT in “Supplementary material.” Estimates of the size and roundness of grains with depth (see “Sedimentology” in the “Methods” chapter [Expedition 330 Scientists, 2012a]) are illustrated in Figure F3 and provided in a supplementary table (see U1375A.XLS in SIZE in SEDIMENT in “Supplementary material”). Subunit IIA
Stratigraphic Subunit IIA is 0.84 m thick and extends from 8.50 to 9.34 mbsf (Fig. F3). Its lower boundary corresponds to the top of underlying multicolor basalt breccia (Subunit IIB; see below). Subunit IIA is composed of heterolithic grain-supported, poorly sorted multicolor basalt conglomerate (Fig. F4). The intercobble spaces include whitish-yellow foraminiferal limestone, sand- to granule-size volcaniclasts, calcite cement, and voids. The limestone is characterized by a complicated white-to-yellow color pattern and local occurrences of black (ferromanganese?) dentrites. The yellow limestone is characterized by higher phosphorus content than the white limestone (see XL3_EVAL.PDF in XRF in “Supplementary material”), supporting alteration of the limestone by postdepositional, probably metasomatic, processes (Murdmaa et al., 1995). The conglomerate clasts are composed of abundant basalt pebbles and cobbles (see “Igneous petrology and volcanology”), a few pebble- and cobble-bearing volcanic sandstones with bioclasts, and rare granule-size pebbles of bioclast limestone. The largest clasts per 10 cm interval are composed of basalt cobbles, and the average clast roundness is angular to subangular. Smaller sized grains are, however, well rounded to angular. The intercobble and interboulder spaces include current structures with a (minor) erosional contact at ~9.28 mbsf (Section 330-U1375A-2R-1, 78 cm), geopetal structures formed by infilling of hemipelagic and volcaniclastic sediments, and possible sediment-flow structures. Distinct limestone poorer in foraminifers was observed in the conglomerate matrix between the erosional contact and underlying Subunit IIB (interval 330-U1375A-2R-1, 78–84 cm). Minor bioturbations were observed in the whitish-yellow foraminiferal limestone uphole (interval 330-U1375A-2R-1, 15–25 cm). The age of Subunit IIA was constrained by foraminiferal and nannofossil assemblages from the early to late Paleocene (see “Paleontology”). Thin section observations (Samples 330-U1375A-2R-1, 26–29 cm [Thin Section 224], and 2R-1, 67–70 cm [Thin Section 225]), showed that the intercobble and interboulder spaces include rare shallow-marine bioclasts (red algae, echinoderms, shell fragments, and bryozoans) and granular calcite cement. Clasts of vitric sandstone composed of altered vesicular glass fragments with altered olivine contain rare dissolved shell fragments and possible red algae. Also, these clasts contain three generations of cement (fringing fibrous zeolite, fringing fibrous calcite, and sparry calcite, in order of formation), and sedimentary textures show they were lithified before reworking. Subunit IIB
Stratigraphic Subunit IIB is 0.77 m thick and extends from 9.34 to 10.11 mbsf (Fig. F3). Its lower boundary was not recovered. Subunit IIA is composed of monolithic matrix-supported poorly sorted multicolor basalt breccia (Fig. F4). The matrix of the breccia is composed of cemented volcanic sandstone without bioclasts. The sediment of Subunit IIB is very altered (see “Alteration petrology”). The larger clasts range in size from granule to cobble, and average clast roundness is angular to very angular. Subunit IIB clasts differ from those of Subunit IIA and include basalt pebbles and cobbles with a jigsaw-fit texture (Fig. F4B; see also “Igneous petrology and volcanology”). Sedimentary infills and cement found along the upper edge of some of the cobbles with a jigsaw-fit texture suggest that the basalt clasts were reworked from an older sedimentary or volcanic deposit during or prior to formation of Subunit IIB. Microscope observations (Sample 330-U1375A-2R-1, 137–141 cm [Thin Section 226]) indicated that the sediment is predominantly composed of altered vesicular glass fragments with rare fresh olivine. Interpretation of sediment at Site U1375Although sediment retrieved from the uppermost part of Hole U1375A was restricted to cuttings found in Core 330-U1375A-1R, our observations and the faunal assemblages (see “Paleontology”) suggest that Unit I represents a pelagic cap similar to those found at Sites U1372 and U1374 on Canopus and Rigil Guyots, respectively. Similar to previous Expedition 330 sites, the pelagic cap at Site U1375 may represent a winnowed residue of ooze initially richer in nannofossils. Unit II in Hole U1375A is interpreted as the uppermost part of an older sedimentary cover on Achernar Guyot. On the basis of a ferromanganese encrustation that occurs at the top of Unit II, the texture of the foraminiferal limestone found in the conglomerate matrix, and the absence of shallow-marine organisms apart from few reworked sediment clasts, we interpret Subunit IIA as a hemipelagic, partly metasomatized interval deposited during drowning of Achernar Guyot. Nannofossils and foraminifers indicate an early to middle Paleocene age of deposition (see “Paleontology”). The basalt breccia of Subunit IIB is interpreted as a debris flow deposit on the basis of its matrix-supported texture and lack of foraminiferal limestone. Only a few cuttings were retrieved from the core catcher of Core 330-U1375B-1R in Hole U1375B, including abundant basalt grains, minor ferromanganese fragments, rare foraminifers, clinopyroxene grains, and limestone fragments with black dentrites. These observations point toward the existence of a similar sedimentary sequence in Holes U1375A and U1375B. |
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